When a few hundred Spaniards landed in what would become Mexico in 1519, they marvelled at its bustling cities. But within four generations that thriving native population was reduced by two-thirds - a phenomenon usually blamed on European diseases to which the natives had no resistance. Now a researcher suggests that most of the damage was done by a local haemorrhagic fever not unlike the Ebola virus.

Estimates of the population before the Spanish conquest range from 6 million to 25 million. By 1600, it was 2 million. Acuna-Soto accepts that epidemics of smallpox in 1520 and measles in 1530 fit the conventional account of imported diseases to which natives had no resistance. But he believes that two other heavy losses of population, in 1545 and 1576, do not fit this account.

Acuna-Soto's conviction that these epidemics - called cocoliztli - were caused by a haemorrhagic fever is partly based on the observations of Philip II's physician. 'Blood flowed from the ears and in many cases blood truly gushed from the nose,' Francisco Hernandez wrote. 'The tongue was dry and black. Enormous thirst. Urine the colours of sea green, vegetal green, and black'.

'The pathology simply does not correspond,' insists Acuna-Soto, of the National Autonomous University of Mexico. 'There are mysterious epidemics all over the world. But never anything of this magnitude, ever.'

Elsa Malvido, a historian from Mexico's National Institute of Anthropology and History, dismisses the idea. She believes the later epidemics were bubonic plague spread by black rats from Europe. 'They were more extreme because they were attacking people with no immunity,' she said.